Aged naturally... to play both older and younger... within a single film...
Given how most films only take a few months to film at most, I don't think natural aging will have enough impact on a character to have a distinct "younger and older". If the film took years to make, then parts would seem really low quality compared to other parts. If the character was only a few weeks older, you wouldn't notice the difference between the accurate age and the reshoots.
Given how most films only take a few months to film at most
Got any sources for that claim? I was always under more of an impression that movies took at least a year and a half to make, if not 2-3 for a longer movie.
Filming isn't the longest part of movie production. They spend a significant time editing, piecing stuff together, getting the audio and color balance right and so on.
For the average movie, the actual shooting of footage usually takes between one and three months. The time depends on the length of the script, but the going is slow. “One or two script pages will take a whole day to film,” says cinematographer Margaret Kurniawan.
I didn't say "make", I said "film". You don't film the actors until the script, sets and costumes are ready, and you can't edit the movie until after you've finished filming.
According to wikipedia, filming for Avengers Endgame started in August 2017 and finished in January 2018, with reshoots later that year. This was a big Marvel production with a ton of big-name actors doing complex action scenes, so I'm pretty sure this is on the high end of how much filming would need to be done.
I imagine the average person would take the phrase "film a movie" as synonymous with "make a movie", even if they are technically distinct. If you're being more specific with your usage than the typical person in your audience, that's on you; don't blame your audience for not understanding.